Landscape scenes and other scenes can have very high contrast to them in typically a top-to-bottom gradient. For example, in a landscape scene, the sky may be much brighter than the land. A photograph of the scene, may have limited detail in the image of the sky with good detail of the land or vice versa.
This problem may be corrected by capturing the image through a graded ND filter that lets through less light at the top of the image and more light at the bottom of the image. Such a filter typically has a neutral density portion at the top of the filter and a clear portion at the bottom of the filter. Between these two portions, the density of the filter changes gradually, so as not to cause the line between the neutral density portion and clear portion to be displayed on the image.
In an electronic imager, Using a graded ND filter prevents the pixels corresponding to the bright sky at the top of the image from saturating while not significantly darkening the pixels corresponding to the land at the bottom of the image thus providing the desired level of detail across the image.